The invention relates to a surface structure and a Fresnel lens which has at least one such surface structure. Furthermore, the invention relates to a tool for production of a surface structure and a method for production of a surface structure or Fresnel lens. Furthermore, the invention relates to the use of a Fresnel lens.
Normal lenses with a large lens diameter and short focal distance are very thick and difficult to manufacture. In order to circumvent this problem, Fresnel lenses are therefore used in multiples. The lens surface is thereby subdivided into small prisms which are disposed adjacently in one plane. Thus an essentially flat lens is produced as can be found in everyday life in the case of overhead projectors or as divergent lens on the rear windows of cars.
In concentrating photovoltaics, Fresnel lenses are used in order to concentrate solar radiation onto small solar cells. It is not the aim thereby to produce as clean an image of the sun as possible (imaging lens system), but rather merely to concentrate as much light as possible onto the solar cell (non-imaging lens system). In many applications and concrete systems, it is also sought to achieve as homogeneous a profile of the radiation strength as possible within the focal spot.
As a result of the small size of the solar cells onto which the light is concentrated in highly-concentrating photovoltaics, great demands are made upon the precision of the Fresnel lenses. At the same time, the Fresnel lenses are subjected to the effects of ambient temperature. In many desert areas, temperatures of significantly below 0° C. in winter are not unusual, whereas in summer the midday temperatures easily exceed 40° C. As a result of the temperature-induced expansion of the materials used in the lens, the refractive index of these materials changes, on the one hand, and the lens is deformed, on the other hand. The effects of temperature hence lead to a Fresnel lens fulfilling its function as concentrator with varying effectiveness, as a function of the temperature thereof, and hence indirectly as a function of the ambient temperature, the radiation and other meteorological parameters, such as e.g. wind strength and direction.
The lens geometry is based on assumptions about the refractive index of the lens material. Because the refractive index is temperature-dependent, the Fresnel lens is hence optimised with respect to a specific temperature, e.g. the average temperature, during operation. Deviations from this temperature lead to the Fresnel lens fulfilling its purpose less well because of the refractive index change associated therewith.
Generally, the original shape used in the production process of the Fresnel lens is designed such that it corresponds to the desired lens structure in current operation. Hence, the negative effect of temperature-induced deformations on the function increases with the temperature difference between operating temperature and production temperature. The production temperature is however generally significantly below (e.g. room temperature) or above (e.g. thermoplastic deformation) the typical temperatures which occur during operation.
In addition, additional deformations occur during the production process of the Fresnel lenses, e.g. due to volume shrinkage. The produced Fresnel lenses are hence no longer a true copy of the tool and do not offer optimal functionality.
In concentrating photovoltaics, at present two material combinations are used for preference:
Fresnel lenses made of polymethylmethacrylate (PMMA), designed as solid lens plate, or Fresnel lenses made of silicone which are applied on a glass plate. A Fresnel lens made of PMMA without inner stresses expands isotropically, i.e. its size changes upon temperature changes during operation but not the proportions. This ideal case in reality seldom occurs however so that also these lenses deform as a result of inner stresses or non-homogeneous temperature distributions.
In DE 29 20 630 A1 and also in U.S. Pat. No. 3,982,822, Fresnel lenses are described which are manufactured from two materials with different coefficients of expansion. In the case of these Fresnel lenses, the thermal expansion according to the previous state of the art was taken into account in the production only from the point of view of durability (see e.g. U.S. Pat. No. 3,982,822). The thermal expansion was classified previously as unproblematic from an optical viewpoint (see DE 29 20 630 A1 and U.S. Pat. No. 3,982,822).
It has however emerged that thermal effects noticeably influence the optical properties of the Fresnel lenses. As a result of the low thermal expansion of glass, the large-area change in shape plays only a subordinate role. The significantly greater thermal coefficient of expansion of silicone leads however to the silicone structure which is significantly more elastic compared to glass being deformed. This deformation takes place on the large scale of individual Fresnel prisms or facets. For example, originally straight-shaped prism edges are thereby deformed. Analogous effects or deformations can thereby be expected in all systems in which the thermal expansion of a substrate material differs from that of a lens material.
Starting herefrom, it is the object of the present invention to eliminate the disadvantages of the state of the art and to provide a surface structure and also a Fresnel lens which can be produced in a simplified manner and nevertheless have very good optical properties.